I agree with what's already been said--silence is deafening enough to someone who listens. Someone who doesn't isn't worth much time.
We can make all kinds of guesses about Y and the whole situation there, but in the end it's not particularly relevant. It only stands that Y was rude for whatever reason, and X tried to explain it away afterwards. X may have been trying to apologise (poorly) or she might have been being snide; either way, the initial response would be the same.
The only other response to X's statement I can think of would be, "what an interesting assumption" or a variation thereupon. But who can possibly be expected to be ready to say something so unexpected and ridiculous.
Or... this really is for only the best actors among, but... I know someone who's brother once said something to her like, "you're my sister, so you have to do *enormous big favour* for me whenever I want," and she just burst into laughter at what he'd said, leaving him dumbstruck. There's nothing like making someone realise how foolish what they've said is to get the point across. But I don't think most people could ever pull this off.